1. Field of the Invention
This invention relates generally to electrical devices which are capable of driving a capacitive load. Specifically, this invention provides a method for improving the efficiency of H-bridge electrical switching devices that drive a capacitive load.
2. Description of the Prior Art
The current state of the art describes an H-bridge 5 configuration which consists of four switches as shown in FIG. 1A. The switches are typically bipolar transistors, MOSFET transistors or other electronic devices known to those skilled in the art. A capacitive load 6 is coupled to the H-bridge. Examples of capacitive loads include, but are not limited to, electroluminescent (EL) lamps, liquid crystal elements and piezoelectric transducers.
The H-bridge functions to convert a DC voltage to an AC voltage for driving the capacitive load by manipulating the switches. Referring to FIG. 1A, the method described by the current state of the art for H-bridges is to close SW1 1 and SW2 2 which charges the capacitive load 6 with a potential equal to Vcs 7. Switches SW1 1 and SW2 2 are then switched open and SW3 3 and SW4 4 are switched closed as shown in FIG. 1B. This has the effect of discharging the capacitive load 6 in one direction and then charging the capacitive load 6 with a potential also equal to Vcs 7, but in the opposite direction. Thus, the capacitive load effectively sees a peak-to-peak AC voltage that is two times that of the DC voltage, Vcs 7. In other words, the capacitive load 6 sees a voltage swing of +Vcs to -Vcs. This method of switching the DC power to the capacitive load on the order of 50 Hz to 5000 Hz leads to inefficient operation because of the peak to peak voltage swing is double that of Vcs.
Therefore, a need existed to provide a method for improving the efficiency of H-bridge electrical switching devices which drive a capacitive load that reduces the peak to peak voltage swing across the capacitive load.